docs: def labels_for_zone(zone):
      '''
      Extract all known zone labels
      - main code
      - keys (postal...)
      - name translation in every supported languages
      '''
      labels = set([zone.name, zone.code] + zone.keys_values)
      for lang in current_app.config['LANGUAGES'].keys():
          with language(lang):
              labels.add(_(zone.name))
      return list(labels)